Imagine if a gallon of milk cost three dollars in your town, but 100 miles away it cost $100, or even $200.

Something similar is happening right now in California with water that farmers use to irrigate their crops. Some farmers are paying 50 or even 100 times more for that water than others who live just an hour’s drive away.

The situation is provoking debate about whether water in California should move more freely, so that it can be sold to the highest bidder.

For all of you who aren’t intimately familiar with the wacky and wonderful world of California water, here’s some background.

California is America’s biggest agricultural state, but its vineyards, orchards and vegetable fields don’t actually get enough rainfall to grow a crop. Some of those fields — notably the “salad bowl” of the Salinas Valley — get their water from wells.

The majority, though, depend on water from far away, mainly from melting snow in the mountains. Dams capture it, pumps and canals distribute it and lawyers argue over who gets to use it.

Some end up with much more water than others.

Let’s start with one of the fortunate ones: Allen Peterson, who grows almonds near the city of Turlock, Calif. A concrete-lined canal full of water runs right past his orchards. “The water’s coming from Lake Don Pedro, on the Tuolumne River,” Peterson explains.

The Turlock Irrigation District started building dams on the Tuolumne more than a century ago. Now, every farm in this district gets a share of the lake’s water. This year, it’s less than usual, but still enough to grow a crop of almonds.

That secure source of water is as much a part of Peterson’s farm as the land itself. It’s also a family legacy. “My grandfather, and even people before him, built this irrigation system,” Peterson says. “He scraped canals, built this thing up. They sacrificed a lot to have this irrigation system. And our land prices have reflected that ever since.”

His land is valuable. The water itself, though, comes cheaply. Peterson is paying the district just under $30 this year for each acre-foot of water. (That’s enough water to cover an acre of land to a depth of one foot.)

Meanwhile, there are farmers not far away, on the other side of California’s Central Valley, who’ve been paying much, much more.

“We’ve had water that’s sold for upwards of $2,000 an acre-foot,” says Sarah Woolf, a farmer and water consultant in Five Points, southwest of Fresno. “It was horrible.”

Woolf and I are standing in an almond orchard that’s alive thanks to that expensive water.

It’s part of the Westlands Water District, which came late to the California water party. It tapped into the state-wide system of aqueducts just 50 years ago. So under California’s water laws, when there’s not enough water for everyone farmers here are the first to be cut off.

They were cut off this year. The owner of this orchard turned to Woolf to help him buy enough water to keep these trees alive. Woolf located a few farmers outside the Westlands area with rights to water that they were willing to sell.

The actual transfer of water was just a matter of aquatic bookkeeping. The sellers gave up their rights to draw some water from California’s aqueducts, and this farmer was able to use that amount of water instead.

But this kind of exchange doesn’t happen very often. In some places, it’s banned. The Turlock Irrigation District, for instance, doesn’t allow farmers to sell any of their $30 water outside the district.

This is why farmers are paying such wildly different prices. The water’s not allowed to move.

Economist Richard Howitt, at the University of California, Davis, believes that this is really unfortunate. Irrigation water should flow more freely to the places where it’s needed most, he says. A free market in water would leave everyone better off. “It should be good for both producers and consumers to have more efficient use of our basic natural resource,” he says.

There are some physical barriers to moving the water around. Very few aqueducts and rivers run between water-rich areas like Turlock, on the eastern side of the Central Valley, and thirsty areas to the west.

But Howitt says that problem could be solved. “With small engineering changes, we could move the water from east to west, from the $20 region to the $2,000 region,” he says.

The emotional and political barriers are more difficult to overcome. Many farmers just don’t want to sell something that’s so central to the life of their community. “If we sold our water off, the jobs would go away here, too. There would be less commerce going on in our county,” says Peterson, the farmer in Turlock.

Howett, the UC Davis economist, says farmers also don’t want to raise any questions about their legal rights to water. “They are worried that if they sold the water, they would be admitting that they didn’t ‘need’ it.”

In that case, others might try to claim it. Environmentalists, for instance, would like more water to flow into California’s rivers and wetlands.

Woolf, the farmer and water consultant in Five Points, says that’s why farmers who do sell water sometimes won’t admit it.

“There’s definitely a [feeling of] ‘hush-hush, I don’t want to talk about it, they’ll take my water away,’ ” she says. “And I don’t blame them for that.”

If California’s water-rich farmers didn’t have to worry that someone might take their water, they might be more willing to sell it, she says. And everybody would benefit.